1. Field of the Invention
The present invention relates to an improvement to a process for the production of a microtip electron source. It makes it possible to improve the uniformity and/or reproducibility of the emission of microtip cathodes and reduce the manufacturing constraints.
2. Discussion of the Background
Emissive microtip cathodes are electron sources more particularly used in the display field, more specifically for flat-face screens. They can also be used in electron guns or vacuum gauges.
FR-A-2 593 953 describes a process for the production of a display means by cathodoluminescence excited by field emission. The electron source is a microtip cathode deposited on a glass substrate and having a matrix structure.
FR-A-2 623 0 13 and FR-A-2 663 462 describe improvements made to said microtip cathode. They more particularly relate to the improvement of the emission uniformity by limiting the current in the tips which emit the most electrons. This result is obtained by introducing a resistor in series with the microtips. This resistor is formed from a continuous or non-continuous resistive layer. FIG. 1 diagrammatically shows a known electron source having microtip missive cathodes described in detail in FR-A-2 623 013. This source has a matrix structure and optionally incorporates a thin silica layer 4 on an e.g. glass substrate 2. On said silica layer 4 are formed a plurality of electrodes 5 in the form of parallel conductive strips serving as cathode conductors and constituting the columns of the matrix structure. The cathode conductors are in each case covered by a resistive coating 7, which can be continuous (except at the ends in order to permit the connection of the cathode conductors to polarization means 20). A silica, electrically insulating coating 8 covers the resistive coating 7. Above the insulating coating 8 are formed a plurality of electrodes 10 also in the form of parallel conductive strips. These electrodes 10 are perpendicular to the electrodes 5 and serve as grids forming the lines or rows of the matrix structure. A resistive coating can optionally be deposited above or below the electrodes 10.
FR-A-2 663 462 recommends the use of electrodes (e.g. cathode conductors) in lattice form, in such a way that the microtips are located in the openings of the lattice of said electrodes. In this configuration the breakdown resistance is no longer mainly dependent on the thickness of the resistive coating, but instead on the distance between the cathode conductor and the microtip.
Another improvement to said microtip cathodes is provided by French patent application 92 02 220 of Feb. 26, 1992, now FR 2,687,839, in the name of the present applicant. These improvements consist of reducing short-circuiting risks between the rows and columns by means of microtips. In order to do this, there is a maximum reduction of the overlap zones of the two series of electrodes, as illustrated in the attached FIGS. 2 and 3.
FIG. 2 is a diagrammatic plan view of the electron source and FIG. 3 a larger scale sectional view along axis III--III of FIG. 2. This matrix structure comprises an e.g. glass substrate 1 and optionally a thin silica coating 6. On said silica coating 6 is formed a series of parallel electrodes 3, each having a lattice structure and serving as cathode conductors, forming the columns of the matrix structure.
The cathode conductors 3 are covered by the silicon resistive coating 9 and by the silica insulating coating 11. Above the insulating coating 11 is formed another series of parallel electrodes also having a perforated, but differing structure designed to minimize the overlap zones with the cathode conductors. These electrodes are perpendicular to the cathode conductors and form the grids, being the rows of the matrix structure.
FIGS. 2 and 3 show in detail one of the grids of the device. The grid 13 has parallel tracks 14 orthogonally intersecting other parallel tracks 15. At the intersections of the tracks 14 and 15, the grid has widened zones 17, which are square here. FIG. 2 shows the overlap zone 16 of the cathode conductor 3 and the tracks 14 and 15 of the grid with a very small surface. The widened zones 17 are located in the center of the meshes forming the cathode conductor.
In the intersection zones of the cathode conductors and the grids, microholes 18 are formed in the thickness of the grid and the insulating coating 11. Microtips 19 are deposited in these holes and rest on the resistive coating 9. A microhole and microtip assembly constitutes an electron microemitter. The microemitters occupy the central regions of the meshes of the lattice of the cathode conductor and the widened, square zones 17 of the grid. In the case shown in FIGS. 2 and 3, each of the meshes of the cathode conductor and each widened grid zone has 16 microemitters.
The dimensions of the microemitters are optimized in order to obtain the best possible emission. It is a question of the diameter of the holes, the geometry of the tips, the thickness of the insulating coating and the thickness of the grid. Thus, the emission current is highly dependent on these dimensions and is inversely proportional to the diameter of the holes. It is of an optimum nature when the holes are circular and decreases when the holes lose this circular shape, e.g. when they become oval. The emission current is also at an optimum when the apex of the tips is located in the thickness of the grid conductor and drops very rapidly when the tips are high and project above the grid and when they are low, their apex remaining below the grid. The position of the apex of the tips is linked with the thickness of the insulating coating in which are etched the holes and with the geometry of the tips, particularly the angle of the cone which they form.
The apex of the tips is located in the grid thickness in exemplified manner when the following conditions are fulfilled:
thickness of the insulating coating 11=1 .mu.m, PA1 diameter of the holes 18=1.3 .mu.m, PA1 molybdenum tips 19 deposited by evaporation or sputtering perpendicular to the surface, PA1 grid thickness in the widened zone=0.4 .mu.m. PA1 an evaluation is made to establish whether the source has an emission which is sufficiently homogeneous and/or reproducible and/or reproducible on another source, PA1 if the emission of the source is considered to be inhomogeneous and/or non-reproducible, a determination takes place of the defects leading to said inhomogeneous emission and which are due to shapes, dimensions or positions of the holes falling outside the tolerances or because the apices of the microtips are not located in the thickness of the grid, PA1 the mask used during the process is corrected in order to render homogeneous and/or reproducible the future sources produced by said process, the correction consisting of modifying the shapes and/or dimensions of at least certain holes in the mask and/or the number of holes in the mask in order to compensate for the previously determined defects by creating supplementary defects and/or holes.
Despite the improvement to the emission uniformity resulting from the successive improvements disclosed in the aforementioned documents, inhomogeneities have been found associated with defects in the structure of the microtip cathodes. These defects can be due to inadequacies of the microemitter production processes. They can also be due to a lack of planarity on the part of the substrate on which the cathode is produced.
Among the defects found reference can be made to the lack of uniformity of the diameter or shape of the microholes or the poor reproducibility of these parameters (diameter, shape) between individual screens and the poor alignment between the emissive tips and the grids, so that certain tips do not emit.
This is due to an inadequate control of the production parameters and/or due to imperfections in the equipments used. The emitted current is constant when the apex of the tips remains within the thickness of the grid and the diameter of the holes is constant. If, in the emissive surface of a cathode or between individual cathodes, the diameter of the holes varies in an uncontrolled manner, or if the tips pass out of the thickness of the grids, the emitted current will vary and the uniformity of emission or its reproducibility is no longer ensured. Thus, the emission is affected if the manufacturing parameters pass outside the admissible tolerance range for obtaining the requisite dimensions for the microemitters.
However, the equipments used for the manufacture of the emissive cathode are not perfect and their performance characteristics are only optimum and reproducible within a certain tolerance. If this tolerance is broader than that of the emissive structure, the emission characteristics are affected. Moreover, certain defects are caused or made worse by the substrate, particularly its lack of planeity.